This invention relates to arrangements for suppressing high-frequency currents in bearings of shafts in treatment apparatus having operating frequencies about 10 kHz.
It is known to short circuit the bearings of shafts in order to avoid radio and television interferences (see, for instance, German Pat. No. 146,636). In connection with electric d-c and a-c machines it is further known to avoid detrimental bearing currents by an insulated construction of the bearing (see, for instance, German Auslegeschrift No. 1 161 992). French Pat. No. 1,217,229 discloses an arrangement in which the bearings are insulated and the shaft is short circuited. The known arrangements, however, are ineffective for high frequency currents above 10 kHz, since, due to the geometric dimensions of the insulation structure of the bearing, especially for larger shaft diameters from 20 mm up, a capacity is obtained, the reactance of which is not high enough in relation to the capacity of the bearing. This brings about an excessively high bearing voltage due to the capacitive voltage division. It has been found that the bearing voltage must not exceed approximately 0.7 V, since, otherwise, fritting takes place in the bearing, which leads to the electroerosive destruction of the bearing. It is furthermore necessary in some applications, on the basis of interference elimination principles as well as for reasons of insulation, to bring the shaft ends of rotating parts in high frequency current carrying apparatus to ground or chassis potential as far as possible. In other applications, the shaft is not to be grounded, so that its electric potential becomes more or less undefined. In such a case, the shunt resistance to the series branch is mainly determined by the coupling capacity between the shaft and the housing and therefore can no longer be considered as small in relation to the bearing impedance.
Bearing currents are a particular problem in certain installations. For example, in capacitive drying installations for paper, the bearing support of the shaft constitutes a particular problem since the bearings are continually being destroyed by the high frequency currents induced by the equipment carrying out the capacitive drying.